Geochemical examination of the active hydrothermal system at Ngawha, Northland, New Zealand: hydrochemical model, element distribution and geological setting

Abstract

The Ngawha geothermal system is the only known high temperature (220-230˚C) system in New Zealand outside the Taupo Volcanic Zone. This current examination integrates new and available geochemical and geological data on the system and surroundings. Ngawha occurs in a Quaternary-Holocene basalt field, within a ENE-trending extensional fault block 15 km in width. The youngest volcanism in the region is associated with this structure. The basaltic activity changed in composition from earlier (? 1.5 to 0.5 m.y.) high-Al, to younger alkali basalt (< 0.5 m.y. to at least 1200 yr b.p.). Crystal fractionation of the alkali basalt magma produced comenditic rhyolite lava, which outcrops as a dome near Ngawha, and is also inferred to have formed an unerupted intrusion, the likely heat source. The geothermal system has developed within pre-existing fault/fracture permeability in basement metasediments (Permian-Jurassic greywacke and argillite), and is concluded to be on the order of 10,000-20,000 years old. The hydrological model developed for the system is of a fault-bounded reservoir within basement rocks, formed of a series of blocks within which fluid migrates in fractures and joints. The reservoir has a low permeability base from silica deposition, and fluid is confined by a caprock of 500-700 m of Cretaceous-Tertiary marine sediments. This allows only vapour discharge at the surface, and minor local leakage of reservoir waters. Recharge to the system is indicated to occur from the N-NE, with subsurface discharge from the reservoir to the SW. Recharging waters are heated during deep circulation (? 3.5 km) and enter the reservoir from faults to the N and on the southern boundary. Vertical displacement of up to 100 m occurs on some of these faults. Most of the 15 wells drilled (usually 1000-1200 m deep) are within the reservoir. The reservoir fluids are slightly acid pH (5.6 at 230˚C) alkali C1 type, but contain high B (800 mg/kg) and NH3 (200 mg/kg). They have a high gas content, largely CO2 (1.2 wt %) and H2S (100 mg/kg). These fluids have ascended in boundary faults, "degassing" during ascent, with the greatest vapour separation in the upper part of the reservoir. The residual fluids then enter the reservoir. Most dissolved constituents are probably derived from high temperature (? 350ºC) leaching of metasediments at depth below the reservoir. Some, however, also have a magmatic component (CO2, S(H2S), N2(NH3), Hg). The fluids have elevated б18O values (+ 5.5 ‰) relative to local meteoric water (-5.5‰), but reservoir rocks have only been depleted c. 1‰. It is concluded the high б18O is derived from rock leaching at depth, a magmatic component and boiling enrichment during fluid ascent. Reassessment of the hydrothermal mineralogy and oxygen isotopes in quartz, show that the system previously contained 260º-280ºC fluids. Tectonic (fault) movement resulted in an inflow of cooler groundwater from the E "flooding" part of the reservoir and reducing temperatures to c. 180ºC. continued inflow of hot water from the N and S, and heat in rocks, has reheated the reservoir to the current measured temperatures (c. 230ºC). The onset of the cool inflow was probably only several thousands of years ago, and it has persisted and produced a zone of fluid mixing across the central part of the reservoir. This inflow can be identified by oxygen and carbon isotopes (in quartz and calcite), fluid chemistry, alteration minerals, and major and trace element chemistry of rocks, as well as downhole temperatures. Temperature inversions have resulted in some parts. The distribution of major, and twenty-six trace elements, was examined in cores and cuttings from twelve geothermal wells, and compared to equivalent non-geothermal lithologies. Distribution was also related to temperature, permeability and mineralogy. Most major elements have been added to reservoir rocks, but there is obvious depletion of K and Al. Of trace elements, Ba, Rb and Th are strongly depleted. Most trace elements typically show trends of major elements with which they are associated, usually by ionic substitution (e.g. Ca-Sr; K-Rb). Zn, for example, is strongly associated with Fe-and Mg-bearing alteration minerals. Some elements can be correlated with temperature, such as increasing Li, Cd, S, Ca, La and Mg. Base metals are typically enriched 30-50% relative to non-geothermal samples. Element associations, are however, often hard to determine due to the limited distribution of alteration (very low water/rock ratios), the occurrence of elements in different mineral phases, and the episodic deposition of hydrothermal minerals. The basement rocks (Waipapa Group) are of quartzo-felds-pathic nature, but have a minor volcanic contribution. Ratios of immobile trace elements (La/V vs Y) appear to be useful in distinguishing whether geothermal samples are greywacke or argillite. Sulphur fugacities of Ngawha fluids are low and S-bearing minerals are not abundant. Sulphide minerals are of limited occurrence, pyrite being the main sulphide (< 5% of rocks) with minor amounts of poorly crystalline pyrrhotite. Both are more common in the upper reservoir-lower caprock, a zone in which boiling occurs. Pyrite is often of earlier (hotter) formation, and pyrrhotite of the recent-current regime. Pyrrhotite is typically monoclinic or monoclinic + hexagonal. Minor arsenopyrite was found locally in a fault intersection; traces of sphalerite and chalcopyrite have been identified. Minor S as a sublimate forms veins at the surface, but hydrothermal SO4 minerals are in trace amounts only. Grains of primary (detrital) barite were identified in metasediments. Minor amounts of As and Sb sulphides occur at the surface in the main thermal zone. Within that area Hg was previously mined both as cinnabar in siliceous lenses, and adsorbed Hg˚ in fine grain sediments. Mercury is transported through the system as Hg˚ vapour. Downhole analyses of cuttings (30-50 m intervals) show Hg has not been leached from rocks in the reservoir, and is stable in pyrite at that temperature range (200˚-250˚C). The decrease of temperatures in the caprock (<150˚C) allows adsorbed Hg˚ to become stable and deposit. Gold and Ag are in low concentration in all geothermal rocks, the highest being Au = 0.07 ppm and Ag = 0.55 ppm. Gold is mostly associated with pyrite and concentrations are higher in the hot inflow zones; it is depleted in the cool inflow, presumably by subsequent dissolution. Silver occurs in pyrite, but also as other phases not identified (possibly in pyrrhotite), and is enriched in the cool inflow. Well discharge silica has relatively elevated concentrations (Au = 0.27, Ag = 13.9 ppm) and is considered analagous to sinter deposits that would form in the absence of a caprock. Sinters forming to the N of the thermal area, contain very low metallic trace elements, as they form from neutral pH HCO3 waters in the caprock. Modelling the hydrology of the overall system and using Au bisulphide solubilities suggests the likelihood that Au (and base metals) have deposited from the fluids in upflow zones before they enter the reservoir. This model appears to be supported by greater mineralisation in well Ng5 samples, in the N of the drillfield.